Low-cost circuit board materials and processes for area array electrical interconnections over a large area between a device and the circuit board

ABSTRACT

An electronic device and coupled flexible circuit board and method of manufacturing. The electronic device is coupled to the flexible circuit board by a plurality of Z-interconnections. The electronic device includes a substrate with electronic components coupled to it. The substrate also has a plurality of device electrical contacts coupled to its back surface that are electrically coupled to the electronic components. The flexible circuit board includes a flexible substrate having a front surface and a back surface and a plurality of circuit board electrical contacts coupled to the front surface of the flexible substrate. The plurality of circuit board electrical contacts correspond to plurality of device electrical contacts. Each Z-interconnection is electrically and mechanically coupled to one device electrical contact and a corresponding circuit board electrical contact.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/435,960, filed May 12, 2003 now U.S. Pat. No. 6,849,935 which claimsthe benefit of priority of U.S. Provisional Application No. 60/379,456,filed May 10, 2002, the contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

This invention is in the field of electronic device circuit boards andinterconnections, and specifically relates to the use ofZ-interconnections with flexible circuit boards.

BACKGROUND OF THE INVENTION

The use of short interconnections normal to the surfaces of circuitboards (Z-interconnections) is one method to desirably create spacesaving multi-layer circuit board configurations. For example, insufficiently complex devices, the number and complexity of the desiredinterconnections may make the use of a multiple layer circuit boarddesign desirable. Matrix array devices, such as are often found inpixel-based detector and display applications, may also desirablyinclude multiple circuit board configurations coupled usingZ-interconnections.

Area array electrical Z-interconnections over relatively large areas(squares of 4 to 6 inches per side) may be particularly desirable tobuild display modules that could be utilized in the construction oflarge-area seamless displays, or relatively large area, high-resolutiondetector arrays. For seamless integration, it is desirable for allelectrical connections from the display panel (device in thisapplication) to the circuit board to be made within the space betweenthe device and the circuit board, because the device is covered withdisplay elements almost all the way to the edges. There may beinsufficient inactive area at the edges of the device for electricalconnections.

Therefore, low cost circuit board materials and processes for formingsubstantially identical Z-interconnections throughout the large areamodule are desirable. Achieving high yields and long-term reliability ofthose interconnections are also desirable.

For example, in the current fabrication of displays based on organiclight emitting diode (OLED) as the active element, it is sometimesconsidered necessary to hermetically seal the circuit board to thedevice. This is because the primary passivation on the device providedby some device manufacturers are not adequate. Therefore, display modulemanufacturers use more expensive rigid inorganic circuit board materialssuch as multi-layer alumina ceramic board that can provide a hermeticcover to the device. A sequential screen printing of conducting (noblemetal) layers and insulating layers on a pre-fired, laser-drilledalumina ceramic is used to achieve the circuit precision needed forlarge area circuits. Due to the relatively lower circuit density ofthese boards, several layers of metallization may be needed toaccomplish the needed circuit routing. These factors result in highmaterials and production cost in making these circuit boards for backpanel applications.

SUMMARY OF THE INVENTION

One embodiment of the present invention is an electronic device andcoupled flexible circuit board. The electronic device is coupled to theflexible circuit board by a plurality of Z-interconnections. Theelectronic device includes a substrate with electronic componentscoupled to it. The substrate also has a plurality of device electricalcontacts coupled to its back surface that are electrically coupled tothe electronic components. The flexible circuit board includes aflexible substrate having a front surface and a back surface and aplurality of circuit board electrical contacts coupled to the frontsurface of the flexible substrate. The plurality of circuit boardelectrical contacts correspond to plurality of device electricalcontacts. Each Z-interconnection is electrically and mechanicallycoupled to one device electrical contact and a corresponding circuitboard electrical contact.

Another embodiment of the present invention is a method of manufacturingthe exemplary an electronic device and coupled flexible circuit board.The exemplary method includes providing the electronic device and theflexible circuit board. A plurality of conductive bumps are formed on atleast one of the electronic device and the flexible circuit board. Foreach device electrical contact, a conductive bump is formed on thatdevice electrical contact, the corresponding circuit board electricalcontact, or both. The plurality of device electrical contacts of theelectronic device and the corresponding plurality of circuit boardelectrical contacts are aligned and the electronic device and theflexible circuit board are bonded together such that the conductivebumps contact the corresponding conductive bumps or electrical contact.The conductive bumps are then cured to form Z-interconnections,electrically and mechanically coupling the device electrical contacts tothe corresponding circuit board electrical contacts.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1A is a side cut-away drawing of an exemplary electronic device andcoupled flexible circuit board, cut along line 1A—1A of FIG. 1B.

FIG. 1B is a top plan drawing of the exemplary electronic device andcoupled flexible circuit board of FIG. 1A.

FIG. 2 is top plan drawing of an exemplary flexible circuit board layoutfor the flexible circuit board of FIGS. 1A and 1B.

FIGS. 3A, 3B, and 3C are side cut-away drawings illustrating exemplaryadvantages of a flexible circuit board, cut along line 1A—1A of FIG. 1B.

FIG. 4 is a flowchart illustrating an exemplary method of manufacture ofthe exemplary electronic device and coupled flexible circuit board ofFIGS. 1A and 1B.

FIGS. 5A-C are side cut-away drawings of alternative exemplaryelectronic device and coupled flexible circuit boards during manufactureaccording to the flowchart of FIG. 4, cut along line 1A—1A of FIG. 1B.

FIGS. 6, 7, 9, 10A and 10B are side cut-away drawing of alternativeexemplary electronic devices and coupled flexible circuit boards, cutalong line 1A—1A of FIG. 1B.

FIG. 8 is a side plan drawing of an additional exemplary electronicdevice and coupled flexible circuit board.

DETAILED DESCRIPTION OF THE INVENTION

The present invention involves low cost circuit board materials andprocesses for achieving high yields and long-term reliability ofelectrical interconnections between a large-area device and a circuitboard.

High interest in OLED displays has led to considerable R & D activity inthis area. Some of this effort has been directed towards hermeticintegral passivations on the front panel. Such a passivation isdesirable to allow the use of non-hermetic back panel materials. Amongthe possible non-hermetic back panel materials low cost organic-basedcircuit board materials offer a number of advantages, such as improvedprocesses for forming substantially identical interconnectionsthroughout the large area module and achieving high yields and long-termreliability of those interconnections. Flexible circuit boards based onpolyimide (for example Kapton from DuPont), polyester (for example Mylarfrom DuPont), and various laminated structures of these families ofmaterials may be particularly desirable. Laminate materials withbuilt-in gas (moisture, oxygen) barrier layers, such as DuPont Mylar 250SBL 300, may be used in applications where having this barrier isdesirable for the back panel.

FIGS. 1A and 1B illustrate an exemplary electronic device 100 andflexible circuit board 102 according the present invention. Substrate104 of electronic device 100 is not shown in FIG. 1B for illustrationpurposes. Although exemplary electronic device 100 shown in theseFigures, as well as in FIGS. 5A-C, 6, 7, 8, 9, 10A, and 10B, is shown asan exemplary six pixel electro-optic array, this in merely illustrativeof a possible electronic device and should not be construed as alimitation. Other electro-optic arrays or electronic devices, such asthose shown in FIGS. 3A and 3B including a plurality of electroniccomponents 302 mounted on drilled substrate 300, may be used as well.

The exemplary electro-optic device shown in FIGS. 1A, 1B, 5A-C, 6, 7, 8,9, 10A, and 10B includes substrate 104, column electrode 106, activematerial 110, passivation layer 111, and row electrodes 120. Deviceelectrical contacts 112 are arranged on the back surface of the deviceto allow electrical coupling of the row and column electrodes to theflexible circuit board. These device electrical contacts may be directlycoupled to the row electrodes and may be electrically coupled to thecolumn electrodes through passivation layer 111 by vias 108.

Desirably substrate 104 is formed of a substantially transparentmaterial such as float glass, quartz, sapphire, acrylic, polyester,polyimide or a laminate of these materials. Column electrodes 106desirably include a substantially transmissive, conductive material suchas indium tin oxide, thin gold, polyaniline, or a combination. Rowelectrodes 120, device electrical contacts 112, and vias 108 aredesirably formed of a conductive material such as aluminum,aluminum-calcium, gold, silver, copper, nickel, titanium, tungsten,platinum, germanium, polyaniline, polyamide, polysilicon, or acombination thereof. It may be desirable for row electrodes 120, deviceelectrical contacts 112, and vias 108 to be formed of the same material.Active material 110 may be formed of semiconductor layers and/or organicpolymer layers to form the light emitting or absorbing portion ofelectro-optic pixel components, such as liquid crystal displays, OLED's,light emitting diodes, and photodetectors.

Flexible circuit board 102 includes flexible substrate 118, circuitboard contacts 116, and a number of electrical traces formed on thefront surface of flexible substrate 118. These electrical traces includecolumn electrical traces 122 and row electrical traces 124, 126, and128. In one exemplary embodiment, the row electrical traces 124, 126,and 128 may be used to provide operational power for different colors ofpixels. For example, row electrical traces 124 may be coupled to redpixels, row electrical traces 126 may be coupled to blue pixels, and rowelectrical traces 128 may be coupled to green pixels.

Flexible substrate 118 may desirably be formed of a flexible organicsubstrate material such as polyester, polyimide or a laminate of thesematerials. Electrical traces 122, 124, 126, and 128 and circuit boardelectrical contacts 116 are desirably formed of a conductive materialsuch as aluminum, aluminum-calcium, gold, silver, copper, nickel,titanium, tungsten, platinum, germanium, polyaniline, polyamide,polysilicon, or a combination thereof. It may be desirable forelectrical traces 122, 124, 126, and 128 and circuit board electricalcontacts 116 to be formed of the same material.

In this exemplary embodiment, electronic device 100 and flexible circuitboard 102 are electrically and mechanical coupled together byZ-interconnections 114. These Z-interconnections are desirably, formedof an electrically conductive material such as indium, a conductivesolder, a conductive thermally-curable epoxy, a conductiveradiation-curable epoxy, a conductive thermoplastic, and/or a conductiveelastomer.

In addition to electrically and mechanically coupling electronic device100 and flexible circuit board 102, Z-interconnections 114 desirablythermally couple electronic device 100 and flexible circuit board 102.Flexible substrate 118 may desirably be very thin <10 mils, therefore,even though the thermal conductivity of many flexible substratematerials may be low, thermal transfer from the front to back surfacesof the flexible substrate may be quite high, but lateral diffusion ofheat may be poor. The relatively high thermal conductivity of theelectrical traces may help with lateral diffusion of heat.

FIG. 2 illustrates an alternative exemplary embodiment of flexiblecircuit board 102, which includes front side thermal conductivity layer200 to assist the lateral heat diffusion of the flexible circuit board.This thermal conductivity layer may desirably be formed of the samematerial as electrical traces 122, 124, 126, and 128. It is alsopossible to form a back side thermal conductivity layer on the backsurface of flexible substrate 118 to accomplish spreading of heat. Thisback side thermal conductivity layer may cover the entire back surfaceof the flexible substrate, or it may be patterned to allow additionalelectrical traces on this side of flexible circuit board 102 and or tochannel heat to certain regions of the flexible circuit board.

FIGS. 3A-C illustrates some of the advantages that may be derived fromthe present invention. In the exemplary embodiments of FIGS. 3A and 3B,electronic device 100 includes drilled substrate 300 with vias 108extending from its front surface to its back surface. This substrate maybe formed of any standard substrate material such as glass, alumina,epoxy resin, fiberglass, polyester, and polyimide. Electronic components302 are mounted on the front surface of drilled substrate 300 andelectrically coupled to electrical contacts 112 formed on the backsurface of drilled substrate 300 though vias 108. These electroniccomponents may include electrical traces, separate components such asresistors, capacitor, and transistors, or integrated circuitry.

FIG. 3A illustrates how exemplary flexible circuit board 102 maydesirably allow for reliable Z-interconnections 114 even in the face ofcamber or unevenness in the back surface of the electronic device. Thisability to overcome camber may allow for less restrictive tolerances inselection of substrates for electronic device 100, possibly lowering thecost of manufacturing and improving yield of these devices.

Also, this relative insensitivity to camber means thatZ-interconnections 114 may desirably be smaller. With a rigid circuitboard, the Z-interconnections should be large enough to compensate forthe anticipated camber. In locations where the camber of both the rigidcircuit board and the electronic device lead to large gaps, theZ-interconnection is desirably formed from a material with a sufficientthickness to cross the gap. When the Z-interconnections are made usingdeformable solder bumps, for example, it is desirable for the deformablebumps from which the Z-interconnections are formed to be at least aslarge as the largest expected gap, which constrains the minimumseparation for the Z-interconnections. In locations where the combinedcambers lead to a narrower gap, the deformable bumps may have too muchmaterial and expand laterally when deformed, further enlarging theminimum separation of the Z-interconnections. This issue may be furtheraccentuated for large area arrays, as larger substrates tend to havelarger cambers.

As FIG. 3A illustrates, this problem is greatly reduced by usingexemplary flexible circuit board 102. This means that the overallthickness of the Z-interconnections may be reduced and that the densityof the Z-interconnections may be greatly increased. ShorterZ-interconnections have less resistance and may lead to a thinner finaldevice. Fine-pitch interconnections are practical with flex circuit backpanels because flex circuits offer high density circuitry, and theinterconnections can be made very small in cross-section. Minimumseparations between the centers of Z-interconnections of <10 mils, oreven <2 mils, may be achieved for large area arrays by using a flexiblecircuit board.

FIG. 3B illustrates the related problem of unevenly sizedZ-interconnections. It may be difficult to precisely control the size ofthe deformable bumps from which Z-interconnections 114 are formed, butdifferently sized bumps lead to differently sized Z-interconnections. Arigid substrate may cause particularly large deformable bumps to expandtoo much laterally, possibly leading to shorts and may not form theZ-interconnections of particularly small deformable bumps. Additionallyit is noted that it may be desirable to intentionally vary the size ofthe Z-interconnections. These issues may be addressed by using exemplaryflexible circuit board 102 as shown in FIG. 3B.

FIG. 3C illustrate the use of exemplary flexible circuit board 102 whenelectronic device 100 includes non-planar substrate 304. It is notedthat although non-planar substrate 304 is shown as substantiallyspherically concave, it could be convex, non-spherically curved, or bentat an angle as well. Such non-planar substrates may be particularlydesirable for use in curved displays or detector, or the fit inparticular spaces within a larger device. Alignment of the circuit boardelectrical connections to the device electrical connections to form theZ-interconnections may be significantly easier for a flexible circuitboard, as the flexible circuit board may be aligned in stages tomaintain the alignment across a large area.

Overall weight reduction of the completed device may also be possiblebecause flex circuit is thinner and weights less than an equivalentceramic, glass, or rigid organic circuit board.

FIG. 4 illustrates an exemplary method of manufacturing an electronicdevice and coupled flexible substrate according to the presentinvention. The method begins with electronic device 100 and flexiblecircuit board 102, step 400. Conductive bumps, or balls, 500 are formedon at least one of the device electrical contact 112 or the circuitboard electrical contact 116 associated with each Z-interconnection 114,step 402. These conductive bumps may be formed of indium, a conductivesolder, a conductive thermally-curable epoxy, a conductiveradiation-curable epoxy, a conductive thermoplastic, and/or a conductiveelastomer. Conductive bumps 500 may be formed on each device electricalcontact 112 and each circuit board electrical contact 116 as shown inFIG. 5A, each circuit board electrical contact 116 as shown in FIG. 5B,each device electrical contact 112 as shown in FIG. 5C, or some deviceelectrical contacts 112 and some circuit board electrical contacts 116(not shown) as long as at least one conductive bump is formed for eachZ-interconnection to be formed. Conductive bumps 500 may be formed usingstandard deposition or screen printing techniques, including sputtering,vaporization, and ink jet methods. It may be desirable to apply flux toconductive bumps formed of conductive solder.

It may be desirable to form the solder bumps on one side of eachZ-interconnection and an organic conductor, such as a conductivethermally-curable epoxy, a conductive radiation-curable epoxy, aconductive thermoplastic, or a conductive elastomer, on the other side.This method may provide better yield than organic conductor bumps alone.It may also be desirable to use the exemplary embodiment of FIG. 5A withconductive bumps 500 on all of the electrical contacts when formingindium or other cold welded Z-interconnections.

As an alternative embodiment, a non-conductive fill layer may also beformed on the back surface of electronic device 100, the front surfaceof flexible circuit board 102, or both, alternative step 404. Thenon-conductive fill layer is desirably formed on a portion of the facingsurfaces on which there are no Z-interconnections. This non-conductivefill layer may desirably be formed of a non-conductive (electrically)organic material such as a non-conductive epoxy, a non-conductivethermoplastic, and a non-conductive elastomer. Electrically conductiveorganic materials such as conductive epoxies, conductive thermoplastics,and conductive elastomers, are often formed by suspending metalparticles in an organic matrix. The non-conductive organic material ofthe non-conductive fill layer may include thermally (but notelectrically) conductive particle within its organic matrix to enhanceits thermal conductivity. It may be desirable for conductive bumps 500and the non-conductive fill layer to include the same organic matrix,but different suspended particles to simplify the curing process of step410.

The non-conductive fill layer may desirably provide for addition thermaland mechanical coupling of electronic device 100 and flexible circuitboard 102. This layer may also assist in forming a hermetic seal aroundelectronic components, such as active material 110, coupled to the backsurface of device substrate 104. Device substrate 104 and/or flexiblesubstrate 118 may also form part of this hermetic seal.

Following either step 402 or step 404, the plurality of deviceelectrical contacts 112 and the corresponding plurality of circuit boardelectrical contacts 116 are aligned with one another, step 406. FIGS.5A-C show exemplary devices at this stage of manufacture according tothe method of FIG. 4. The different embodiments are based on thelocations in which conductive bumps 500 are formed in step 402.

Electronic device 100 and flexible circuit board 102 are then pressedtogether until each conductive bump contacts either the correspondingconductive bump (the exemplary embodiment of FIG. 5A) or electricalcontact (the exemplary embodiments of FIGS. 5B and 5C), step 408. Toensure proper contact between the conductive bumps and the correspondingconductive bump or electrical contact, flexible circuit board 102 may bepressed using a surface, such as a rubber sheet over a hard surface,with sufficient elasticity to allow relatively even pressure over thesurface even as flexible circuit board 102 flexes to desirably conformto the back surface of electronic device 100. Alternatively, anisostatic lamination method may be used to press electronic device 100and flexible circuit board 102 together, using a flexible compressingmembrane and a pressurized liquid, such as water. Isostatic laminationmay be particularly useful exemplary embodiments in which the backsurface of electronic device 100 is significantly non-planar, and/orsignificant deformation of the conductive bumps is desirable.

Conductive bumps 500 are then “cured” to form the Z-interconnections114, step 410. The non-conductive fill layer may also be “cured” to formnon-conductive fill 800, as shown in FIG. 8, at this step if alternativestep 404 is used. The means of “curing” the conductive bumps (andpossibly non-conductive fill layer) depends on the material from whichthey are formed, and may include, for example, heating or irradiating atleast the conductive bumps.

It is contemplated that steps 406, 408, and 410 may be performed instages with only a subset of the Z-interconnections being aligned,contacted, and cured at one time. This “piece meal” method may allow forimproved yield of large area array devices, by allowing alignmentcorrections between sequential operations across the large surface areaand numerous Z-interconnections.

For indium, or other cold welded Z-interconnections, the curing processinvolves applying sufficient pressure to electronic device 100 andflexible circuit board 102 to deform and cold weld conductive bumps 500into Z-interconnections 114. This pressure may be applied uniformly or apressor such as a roller may move across the back surface of theflexible substrate. This moving pressor method may be particular usefulwhen large camber and/or non-uniformity of Z-interconnection size isexpected, or a non-planar substrate is used.

Conductive bumps formed of conductive solder may be cured intoZ-interconnections by using standard solder reflow techniques. It isnoted that although a solder interconnection may be preferred to achievehigh electrical conductivity, the desirable use of flux to facilitatesolder wetting and the relatively high temperatures needed for solderreflow may be detrimental to the device. Therefore it may be desirablein some applications to combine a conductive solder bump on oneelectrical contact with a conductive organic material on the otherelectrical contact to form a hybrid Z-interconnection. These exemplaryhybrid Z-interconnections are cured according to the type of conductiveorganic material used.

To cure conductive bumps formed of conductive epoxies or elastomers itis desirable to press electronic device 100 and flexible circuit board102 together to desirably deform the conductive epoxy or elastomer bumpsinto the shape of the Z-interconnections. For radiation-curableconductive epoxy, the deformed bumps are then irradiated with theappropriate light, i.e. UV for UV-curable epoxies, to harden the epoxy.Thermally-curable conductive epoxy bumps are heated to their curingtemperature and allowed to harden. Conductive elastomerZ-interconnections are held in place until the elastomer material hasset.

Conductive thermoplastic bumps are cured by heating the bumps to abovethe softening temperature of the thermoplastic. Electronic device 100and flexible circuit board 102 are then desirably pressed together todesirably deform the conductive thermoplastic bumps into the shape ofthe Z-interconnections. The Z-interconnections are then cooled to belowthe softening temperature to harden the Z-interconnection.

If a non-conductive fill layer was formed in alternative step 404, thislayer may be cured using the same method as the corresponding conductivebump material. If identical organic matrices are used, the curingparameters may be almost identical, greatly simplifying the curingprocess.

With the possible exception of the embodiment using conductive solderZ-interconnections, which may require high reflow temperatures, thismethod does not introduce any stresses to the glass device panel duringassembly. In addition, when the module is thermal cycled, thermalexpansion mismatch between the glass front panel and the flex circuitdoes not lead to significant stresses because of the relatively lowelastic modulus of the flex circuit.

It is noted that, flexible substrate 118 may have a thermal expansioncoefficient which is significantly different from the thermal expansioncoefficient of electronic device 100 and they may be at differenttemperatures, which may lead to large differences in thermal expansion.Differences in the thermal expansion of the electronic device and arigid circuit board are largely absorbed by the elasticity of theZ-interconnections, but this may lead to failure of some of theZ-interconnections. The relatively much larger elasticity of flexiblecircuit board 102 compared to standard rigid circuit boards results inless strain on the Z-interconnections due to thermal expansiondifferences.

FIG. 8 illustrates two additional alternative features to reducemechanical strain on the Z-interconnections due to thermal cycling.Non-conductive fill 800 may desirably improve thermal coupling betweenelectronic device 100 and flexible circuit board 102, thereby decreasingtheir thermal gradients. Non-conductive fill 800 may be formed asdescribed above in relation to the exemplary method of FIG. 4, or it maybe formed by using a back fill technique after Z-interconnections 114have been cured. It is noted that the Z-interconnections andnon-conductive fill may alternatively be formed together using ananisotropic conductive adhesive disposed between electronic device 100and flexible circuit board 102.

The second alternative exemplary feature shown in FIG. 8 is laminatedsubstrate 802. Desirably, laminated substrate 802 has a thermalexpansion coefficient approximately equal to the thermal expansioncoefficient of device substrate 104. This substrate may desirably belaminated to flexible substrate 118 before or after the curing ofZ-interconnections 114, depending on the interconnection tolerance anddensity desired. Laminated substrate 802 may reduce strain onZ-interconnections 114 and electronic device 100 by reducing differencesin lateral thermal expansion between electronic device 100 and flexiblecircuit board 102. Also laminated substrate 802 may improve the lateraldiffusion of heat in flexible substrate 118.

FIGS. 6, 7, 10A and 10B illustrate several alternative exemplaryembodiments of flexible circuit board 102. In FIG. 6 exemplary flexiblecircuit board 102 includes extended flexible substrate 600 and heatsinks 604. Extended flexible substrate 600 may include a connectorportion 602 that extends beyond the edge of electronic device 100.Electrical traces 122, 124, 126, and 128 may be extended along thisportion of the flexible substrate to provide easily accessibleconnections to the flexible substrate from off of the device. Thisfeature may be particularly useful for tiled optical displayapplications in which it is desirable for electronic device (displayelement array) 100 to butt directly up to an adjacent tile. Because ofits flexibility, connector portion 60 of extended flexible substrate 600may extend behind the adjacent tile and allow connection to off-devicecircuitry without interfering with the adjacent tiles.

Heat sinks 604 may be mounted on the back surface of the flexiblesubstrate to assist with heat dissipation. Due to high heat transferthrough the relatively thin, flexible substrate these heat sink may havea significant effect.

FIG. 7 illustrates two more alternative features that may be added toflexible circuit board 102, dual-sided flexible substrate 700 withelectrical traces on both the front and back surfaces and back surfacemounted electronics 704. A subset of front surface electrical traces122, 124, 126, and 128 may be electrically coupled to a subset of theback surface electrical traces by wirebond 702 and or vias in theflexible substrate (not shown). Back surface mounted electronics 704 aredesirably electrically coupled to the backside electrical traces.Exemplary back surface mounted electronics may include integratedcircuits, memory circuitry, power supply circuitry to provideoperational power for the electronic components of electronic device100, control circuitry to control the electronic components, andanalysis circuitry to analyze output signals from the electroniccomponents.

FIGS. 9, 10A and 10B include the additional feature of a flexiblecircuit board with an elevated portion which may be used to mountadditional circuit board electronic components 904 similar to backsurface mounted electronics 704 of FIG. 7. Exemplary flexible circuitboard 102 of FIG. 9 includes Y-shaped flexible circuit board 900.Circuit board electronic components 904 may desirably be mounted toelevated portion 902 to provide a degree of thermal, and possiblyelectrical and electromagnetic, isolation from the rest of Y-shapedflexible circuit board 900. Thermal isolation may be improved bylengthening this elevated portion. Heat sinks may be added to elevatedportion 902 to remove heat directly from electronic components 904.

Exemplary dual-sided folded flexible substrate 1000 in FIG. 10A andexemplary single-sided folded flexible substrate 1002 in FIG. 10Bachieve this isolation by folding the flexible substrate away fromelectronic device 100 and then back between at least a pair ofZ-interconnections to create folded elevated portion 1004. These foldedflexible substrates may desirably be formed using the alternative “piecemeal” method of manufacturing described above with reference to FIG. 4.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

1. A method of manufacturing an electronic device, which includes asubstrate having a back surface and a first thermal expansioncoefficient, a plurality of electronic components coupled to thesubstrate, and a plurality of device electrical contacts coupled to theback surface of the substrate and electrically coupled to the pluralityof electronic components, and a coupled flexible circuit board, whichincludes a flexible substrate having a front surface and a back surface,and a plurality of circuit board electrical contacts coupled to thefront surface of the flexible substrate corresponding to plurality ofdevice electrical contacts, comprising the steps of: a) providing theelectronic device; b) providing the flexible circuit board; c) forming aplurality of conductive bumps on at least one of the electronic deviceand the flexible circuit board, for each device electrical contact, aconductive bump formed on at least one of that device electrical contactand a corresponding circuit board electrical contact; d) aligning theplurality of device electrical contacts of the electronic device and thecorresponding plurality of circuit board electrical contacts; e)pressing the electronic device and the flexible circuit board togethersuch that at least one of the plurality of conductive bumps spans thegap between each device electrical contact and the corresponding circuitboard electrical contact; and f) curing the plurality of conductivebumps to form a plurality of Z-interconnections electrically andmechanically coupling the plurality of device electrical contacts to thecorresponding plurality of circuit board electrical contacts.
 2. Themethod of claim 1, wherein the plurality of conductive bumps are formedof at least one of indium, a conductive solder, a conductivethermally-curable epoxy, a conductive radiation-curable epoxy, aconductive thermoplastic, and a conductive elastomer.
 3. The method ofclaim 1, wherein: the plurality of conductive bumps are indium bumps;and step (f) includes the step of pressing the electronic device and theflexible circuit board together to deform the indium bumps and cold weldthe plurality of device electrical contacts to the correspondingplurality of circuit board electrical contacts.
 4. The method of claim1, wherein: the plurality of conductive bumps are conductive solderbumps; and step (f) includes the steps of: f1) heating the conductivesolder bumps to at least a melting point temperature; and f2) formingthe plurality of Z-interconnections by solder reflow.
 5. The method ofclaim 1, wherein: the plurality of conductive bumps include a pluralityof conductive thermally-curable epoxy bumps having a first curingtemperature; and step (f) includes the steps of: f1) pressing theelectronic device and the flexible circuit board together to deform theplurality of conductive thermally-curable epoxy bumps; and f2) heatingthe plurality of deformed conductive thermally-curable epoxy bumps to atleast the first curing temperature.
 6. The method of claim 5, wherein:step (c) further includes the step of forming a non-conductivethermally-curable epoxy fill layer on at least one of a portion of theback surface of the electronic device and a portion of the front surfaceof the flexible circuit board, the non-conductive thermally-curableepoxy fill layer having a second curing temperature approximately equalto the first curing temperature of the conductive thermally-curableepoxy bumps; and step (f2) further includes heating the non-conductivethermally-curable epoxy fill layer to at least the second curingtemperature.
 7. The method of claim 1, wherein: the plurality ofconductive bumps include a plurality of conductive radiation-curableepoxy bumps; and step (f) includes the steps of: f1) pressing theelectronic device and the flexible circuit board together to deform theplurality of conductive radiation-curable epoxy bumps; and f2)irradiating the plurality of deformed conductive radiation-curable epoxybumps.
 8. The method of claim 7, wherein: step (c) further includes thestep of forming a non-conductive radiation-curable epoxy fill layer onat least one of a portion of the back surface of the electronic deviceand a portion of the front surface of the flexible circuit board; andstep (f2) further includes irradiating the non-conductiveradiation-curable epoxy fill layer.
 9. The method of claim 1, wherein:the plurality of conductive bumps include a plurality of conductivethermoplastic bumps having a first softening temperature; and step (f)includes the steps of: f1) heating the plurality of conductivethermoplastic bumps to at least the first softening temperature; f2)pressing the electronic device and the flexible circuit board togetherto deform the plurality of conductive thermoplastic bumps; and f3)cooling the plurality of deformed conductive thermoplastic bumps tobelow the first softening temperature.
 10. The method of claim 9,wherein: step (c) further includes the step of forming a non-conductivethermoplastic fill layer on at least one of a portion of the backsurface of the electronic device and a portion of the front surface ofthe flexible circuit board, the non-conductive thermoplastic fill layerhaving a second softening temperature approximately equal to the firstsoftening temperature of the conductive thermoplastic bumps; step (f1)further includes heating the non-conductive thermoplastic fill layer toat least the second softening temperature; and step (f3) furtherincludes cooling the non-conductive thermoplastic fill layer to belowthe second softening temperature.
 11. The method of claim 1, wherein:the plurality of conductive bumps include a plurality of conductiveelastomer bumps; and step (f) includes the steps of: f1) pressing theelectronic device and the flexible circuit board together to deform theplurality of conductive elastomer bumps; and f2) holding the electronicdevice and the flexible circuit board together until the plurality ofdeformed conductive elastomer bumps are set.
 12. The method of claim 11,wherein: step (c) further includes the step of forming a non-conductiveelastomer fill layer on at least one of a portion of the back surface ofthe electronic device and a portion of the front surface of the flexiblecircuit board; and step (f2) further includes holding the electronicdevice and the flexible circuit board together until the non-conductiveelastomer fill layer is set.
 13. The method of claim 1, wherein at leastone of the plurality of conductive bumps has a diameter of less than 5mils.
 14. The method of claim 1, further comprising the step of: g)laminating a rigid substrate to at least a portion of the back surfaceof the flexible substrate of the flexible circuit board, the rigidsubstrate having a second thermal expansion coefficient approximatelyequal to the first thermal expansion coefficient of the substrate of theelectronic device.
 15. The method of claim 1, wherein: the plurality ofconductive bumps include a plurality of conductive solder bumps and aplurality of conductive organic bumps, for each device electricalcontact; a conductive solder bump formed on one of that deviceelectrical contact and a corresponding circuit board electrical contact;and a conductive organic bump formed on a remaining one of that deviceelectrical contact and the corresponding circuit board electricalcontact.
 16. The method of claim 15, wherein: the plurality ofconductive organic bumps are a plurality of conductive thermally-curableepoxy bumps having a first curing temperature; and step (f) includes thesteps of: f1) pressing the electronic device and the flexible circuitboard together to deform the plurality of conductive thermally-curableepoxy bumps against the plurality of conductive solder bumps; and f2)heating the plurality of deformed conductive thermally-curable epoxybumps to at least the first curing temperature.
 17. The method of claim15, wherein: the plurality of conductive organic bumps are a pluralityof conductive radiation-curable epoxy bumps; and step (f) includes thesteps of: f1) pressing the electronic device and the flexible circuitboard together to deform the plurality of conductive radiation-curableepoxy bumps against the plurality of conductive solder bumps; and f2)irradiating the plurality of deformed conductive radiation-curable epoxybumps.
 18. The method of claim 15, wherein: the plurality of conductiveorganic bumps are a plurality of conductive thermoplastic bumps having afirst softening temperature; and step (f) includes the steps of: f1)heating the plurality of conductive thermoplastic bumps to at least thefirst softening temperature; f2) pressing the electronic device and theflexible circuit board together to deform the plurality of conductivethermoplastic bumps against the plurality of conductive solder bumps;and f3) cooling the plurality of deformed conductive thermoplastic bumpsto below the first softening temperature.
 19. The method of claim 15,wherein: the plurality of conductive organic bumps are a plurality ofconductive elastomer bumps; and step (f) includes the steps of: f1)pressing the electronic device and the flexible circuit board togetherto deform the plurality of conductive elastomer bumps against theplurality of conductive solder bumps; and f2) holding the electronicdevice and the flexible circuit board together until the plurality ofdeformed conductive elastomer bumps are set.